With the increasing pace of advances in electronics there has been a corresponding increase in the need for electrochemical devices that provide the energy density, efficiency and safety to power advanced electronic devices, especially portable electronic devices, while still being economically viable. Older battery configurations are often unsuitable to meet these increased demands. Out of environmental and efficiency concerns, the reach of electricity providing devices has been expanded to new areas including hybrid electric vehicles. Ideally, an electrochemical device will provide high current density, decrease the internal resistance of the battery and effectively manage the thermal output of the electrochemical device to increase the longevity of the device.
These features can be achieved by providing massive and/or large surface-area connections between electrodes and cell current collectors, and specifically between cells in a battery. Generally to preserve high specific energy and power, W/kg, Watt-hours per kilogram (Wh/kg) and, Watts per kilogram (W/kg), present technologies and devices fall far short of these goals. A second critical feature of the high power device is internal heat removal. High power to external circuitry generally generates a like amount of energy as heat in short time duration internal to the cell. Excessive temperature rise will destroy (e.g. melt the microporous polymer separator or autoignite the flammable organic electrolyte) or significantly shorten the useful life of the Li-ion cell.
An electrochemical device comprised of cathode and anode electrodes physically exposed to an electrolyte can generically be used to convert between chemical and electrical energies. A housing can enclose these electrode and electrolyte components, and can even seal them from the atmosphere. Batteries, fuel cells and capacitors are but a few such specific electrochemical devices to which this invention relates.
As the electrical power in terms of voltage and/or amperage of each pair of cathode and anode electrodes (or cell) is generally small, many separate pairs of cathode and anode electrodes or cells can be used in a single housing. Current collectors are generally used to electrically interconnect the cells, in parallel and/or in series, to provide usable voltage and amperage outputs at exposed terminals on the electrochemical device.
The electrochemical device performs usable work when ions pass between the electrodes of each cell via the electrolyte, and when electrons concurrently pass through each cell via the electrodes. The generated voltage per cell is predetermined by the electrochemical reaction of the component materials used, and the generated amperage and/or power available is dependent on the configurations and masses of these active components.
The specific output energy of the device can be provided in terms of watts-hours per device weight, and the specific output power of the device can be provided in terms of watts per device weight. Output values of existing electrochemical devices are typically small fractions of theoretically possible output values, because of internal resistances and other inefficiencies (hardware mass and volume).
The resistance to ion conduction between the electrode elements is one major source for internal power loss. Such resistance, R, can be theoretically determined with the expressionR=ρ1/Awhere:    “ρ” (rho) is the impedance value of the electrolyte;    “1” is the thickness of the electrolyte; and    “A” is the interfacial contact area between the electrode elements and electrolyte.
The ionic-impedance value, ρ, is not easily subject to modification and is not effective as a design parameter. Designers of electrochemical devices thus strive to reduce the electrolyte thickness “1”, and to increase the interfacial contact area “A” between the electrode elements and the electrolyte.
Different configurations of the cathode and anode electrodes, electrolyte separation, and the current collection have been proposed. For example, a cathode electrode band can be zig-zagged to define separate compartments for holding electrolyte, and inserted with elongated rod-like anode electrodes into the electrolyte spaced from the cathode electrode. The interfacial contact area “A” effectively is less than the overall surface area of the anode rods, as some rods oppose one another rather than the cathode.
Also, a zig-zagged, folded separator band can define opposing compartments for holding and isolating plate-like cathode and anode electrodes, with electrolyte engulfing all of these components. In an alternative design, each cell can be formed with C-shaped electrodes and a Z-shaped separator sandwiched therebetween. Alternatively, a separator band having electrically conductive surfaces can be folded and sandwiched separate sets of respective plate-like cathode and anode electrodes between the separate oppositely facing folds. A “jellyroll” cell can be formed by coiling a preformed assembly of cathode and anode electrodes and a separator on itself, to yield a cylindrically shaped electrochemical device, with the face-to-face electrodes and sandwiched electrolyte and separator structures, increasing the interfacial contact area “A” between the electrodes.
However, the very breadth of the facing electrodes and sandwiched electrolyte and separator raise another cause of concern, namely the structural sufficiency during assembly and during operation to maintain and support the electrode elements physically separated. This includes withstanding thermal expansion and contraction forces of the cell components during operational temperature changes, such as packing the cell into a box-like housing. Increasing the thickness of the sandwiched electrolyte and separator to provide needed strength and/or durability also increases the ion-conducting electrolyte thickness “1”, offsetting benefits obtained by increased interfacial contact area “A”.
Current collectors used in these cell arrangements add significant weight, and thus reduced specific cell energy and power outputs. For example, isolated conductors are generally connected to the electrodes and routed along extended paths independently of the electrodes to the external terminals. These conductors must carry the full cell current, and thus must be of sufficient mass and cross-section to keep internal resistance manageably low. For a typical battery design of connected terminals, electrode tab/current collector/cell terminal resistance/battery terminal resistance can account for a 50% reduction in battery power output from theoretical capability. Generally, massive connectors are used to avoid power loss for high powered batteries.
Also, these cell arrangements provide electrodes of limited size and/or thickness, limiting the quantities of usable electrode materials and thus limiting maximum cell storage energy and/or operating cycle-life, particularly for rechargeable cells.
The dilemma of these designs is that power gains obtained by increasing the interfacial electrode area “A” across the electrolyte generally are typically offset by increased electrolyte thickness “1”, and the weight and volume of the current collectors reduce specific energy and power outputs. Power can be increased, but only at the expense of reduced energy storage capacity per weight and volume and at increased costs due to needed additional hardware. High interfacial area “A” of the spirally wound “jellyroll” configuration merely trades off usable power against the energy density; but minimum separator thickness is needed for cell durability and cycle-life. Existing bipolar cell arrangements do not escape this power and energy trade off dilemma; nor do fuel cell electrochemical devices.
The rolled-ribbon cell technology disclosed in U.S. Pat. No. 5,219,673 has made great strides achieving enhanced power density for electrochemical devices. Specifically applied to Li/organic-based electrolyte chemistries, improved batteries are formed using the stackable disk-shaped cells to realize near optimum power capability from these cells. Further objectives of batteries for high-pulse power requirement, such as hybrid electric vehicles and power tools, is to continue to reduce battery cost and increase durability. These Li/organic-based electrolyte battery chemistries, although exhibiting quite high voltage (3–5 volts), have relatively low current density capabilities. One limiting factor is the attempted use of relatively-thin components, i.e. the electrode and separator layers. A practical device requires a lot of active area. For example, with peak current density of 10 mA/cm2, it can require 1000 cm2 active area to achieve 10A. For hybrid electric vehicles, the required current is on the order of 100A at 200–400 volts (equivalent to 20–40 kW).
A further dilemma is the large number of small cells required to form such batteries. A major power loss (internal heat generation) is the consequence of batteries with large numbers of small cells (e.g. 1 Ampere-hour (Ah) capacity as in the 18650 cell). More recently larger cells (10 Ah) have used a prismatic configuration. These cells have broad electrodes with multiple tabs connected to a traditional terminal connection. These prismatic cells are hard-wired together (terminal-to-terminal) in a rectangular box. Nonetheless, this arrangement of substantially larger cells can still sacrifice 50% of the theoretical power of the cell chemistry.
However, previous button type cells, typically having very small capacity of 5–50 milli-Ah, lacked ease or consistency of battery assembly and/or distribution of high currents through the cell to the exterior terminals possibly due to the limited conductor paths of hardware components. A hybrid vehicle battery would require hundreds of thousands of these cells.
Thus there is continuing and persistent need for electrochemical devices which have high energy density, provide high power output and approach the theoretical limit for electrical power output.
A Li/organic-based electrolyte battery for high power applications, such as hybrid electric vehicle, must also incorporate features to enhance safety and battery longevity. As there are battery operation and degradation conditions that generate internal gas pressure, there needs to be noncatastrophic, cost effective means to relieve the gas pressure. The typical means is to include a rupture disc on the housing of the Li-ion cell. Rupture of a disc housing causes irreversible failure of that battery, and if a disc ruptures electrolyte may escape to further degrade the battery.
Thermal management is critical to long life of Li-ion batteries in retaining battery capacity particularly due to electrolyte degradation. Batteries capable of generating tens of kW must deal with a like amount of heat generation. Under high pulse power, heat is generated at the electrode/separator interface due to limited ionic conduction. For the conventional jelly-rolled cell, the most direct path for heat loss is across the layers of heat sensitive microporous polymer. Excessive temperature within the cell will locally shutdown the microporous polymer and higher temperatures result with further abuse. Excessive abuse can lead to auto-ignition of organic electrolyte.